1. Field of the Invention
The invention relates to methods of generating structures via photo-resist techniques and somewhat more particularly to a method of generating non-crazed structures in x-ray or electron-beam sensitive resist layers so as to achieve very high aspect ratios after proper irradiation and development.
2. Prior Art
Very fine structures having dimensions in the micron and sub-micron range are required in the manufacture of integrated circuits and components for surface wave and microwave applications. Reproduction techniques, such as electron lithography or x-ray lithography, are particularly interesting for generating such structures because they function with significantly shorter wave lengths than does traditional photo lithography and, thus, provide better resolution. Characteristic for such techniques of electron or x-ray lithography are resist masking layer thicknesses up to approximately 3 .mu.m. Such relatively thin layer thicknesses typically suffice for subsequent work steps, such as etching, vapor-deposition, etc.
A further possible use of electron or x-ray lithography is in galvanic generation of flat metal structures having extremely fine structural dimensions. In this usage, the masking layers, which laterally limit the galvanically precipitated metal, must have a somewhat greater thickness than the molded plastic part. It is the goal of this technique, with the thickest possible masking or cover layer (greater than 100 .mu.m) to achieve vertical edges at those segments which have been uncovered through development, given the smallest possible structural dimensions.
One of the principle difficulties in industrial application of x-ray lithography for generating galvanically precipitated flat surface structures or parts is the production of stress and crack-free cover or masking layers. With the use of a PMMA (polymethylmethacrylate-which is a preferred material used in x-ray lithography) resist as the cover layer, tests have shown that stress cracks already appear in layers after development begins with a PMMA layer thickness of about 3 .mu.m.
A further difficulty of a more general nature is that the non-irradiated areas of a PMMA layer are also partially erroded or dissolved during development (i.e. are subject to dark errosion). This causes a reduction in structural resolution and thus to a diminution of the aspect ratio (i.e. ratio of width to height).